# How to Vibe Code Your Own Grammarly (and Stop Paying for It)

> AI-powered communication and productivity platform

- Site: https://grammarly.com
- Category: Productivity / AI Writing
- Platforms: Web app, Browser extension, macOS app, Windows app, iOS app, Android app
- Verdict: **Serious undertaking** (42/100 vibecodeable)
- Estimated effort: 6-8 weeks of intensive development

## Verdict

Build a web-based text editor subset with AI grammar checks instead of paying, but keep paying for the universal browser extensions and native apps.

Replicating Grammarly's core web editor dashboard with an AI grammar checker and paragraph rewriter is a solid multi-week coding effort. However, cloning the real-time universal browser extensions that inject inline underlines and floating suggestion widgets into arbitrary web inputs (like Gmail, Google Docs, and custom enterprise forms) without breaking host page scripts is an engineering nightmare. Furthermore, running real-time keystroke checks over LLM infrastructure without incurring ruinous API bills requires building a sophisticated hybrid routing proxy.

### What you can't replicate

- Universal browser extension integration across every active web input field on the internet
- 17 years of proprietary grammar rules, datasets, and linguistic heuristics
- Enterprise security certifications, non-training guarantees, and SOC 2 compliance
- The full acquired Superhuman and Coda productivity suite ecosystem

## What it does

Real-time writing assistance, spelling and grammar corrections, tone adjustments, full-sentence rewrites, plagiarism checking, and generative AI features.

### Core features

- Real-time DOM text monitoring and inline correction highlighting
- Grammar, spelling, and punctuation check engine
- Tone detection and adjustment suggestions
- Full-sentence paragraph rewriting via LLM
- Plagiarism checking against document corpus
- Generative AI chat assistant and prompt execution
- Cloud document editor dashboard
- Browser extension surface area for injecting widgets into third-party inputs

## The business

### Pricing

- Free: $0 USD / month
- Pro: $12 USD / member / month
- Enterprise: Custom

### Funding

$1.7B+ raised.
- Series A (2017) - $110M
- Growth Round (2021) - $200M at $13B valuation
- Growth Financing (May 2025) - $1B non-dilutive via General Catalyst
Investors: General Catalyst, IVP, Spark Capital, Baillie Gifford, BlackRock, Breyer Capital

Founded 2009.
Team size: 1,000+.

## The hard parts

- Real-time text streaming and debounce architecture across arbitrary DOM nodes without memory leaks or UI lag
- Cross-platform distribution (4 major browser extensions, native desktop wrappers, mobile keyboards, web editor)
- Cost-aware hybrid LLM routing (fast heuristics for basic grammar, heavy models for complex generative rewrites)
- Enterprise security compliance (non-training guarantees, confidential mode, SOC 2 Type II)

## How to vibe code Grammarly

### Prerequisites

- Node.js (free): Runtime environment for building and executing the web application stack.
- GitHub (free): Source control and deployment pipeline integration.
- Anthropic API Key (pay-as-you-go): Provides frontier language models for paragraph rewrites and advanced tone analysis.

### Recommended AI tools

- Claude Code: Best-in-class agentic CLI tool for scaffolding multi-file full-stack apps and writing complex parsing logic.
- Cursor: AI-native code editor for reviewing fine-grained UI diffs in the web editor dashboard.

### Stack

- Frontend: Next.js
- Backend: Next.js Server Actions / API Routes
- Database: Neon
- Auth: better-auth
- Payments: None (personal clone)
- Other: Tailwind CSS, TipTap / Prosemirror Editor, Anthropic API

### Hosting

- Vercel (Hosting the Next.js web application frontend and API routes): $0-20/mo
- Neon (Serverless Postgres database storing user documents and edit history): $0/mo

### Build guide

1. **Project Scaffolding and Database Schema** — Initialize the Next.js project with Tailwind CSS, configure better-auth with a local SQLite/Neon Postgres adapter, and set up database schemas for users, documents, and writing stats.

```
Create a new Next.js project using TypeScript and Tailwind CSS. Install better-auth and configure database connections to Neon Postgres. Set up database migrations for users, documents (storing title, content, word count, readability scores), and ai_prompts logs. Implement a clean dashboard layout with a sidebar for managing documents and tracking daily writing metrics.
```

2. **Rich Text Editor Dashboard Integration** — Integrate a modern rich text editor (such as TipTap or ProseMirror) into the web application to serve as the core Grammarly Editor clone interface.

```
Install TipTap rich text editor in the Next.js application. Build a custom editor toolbar supporting headings, bullet lists, bold, italic, and inline word/character counters. Implement auto-saving of document content to the Neon Postgres database with a debounce timer and visual save status indicator ('All changes saved' vs 'Saving...').
```

3. **Real-Time Grammar & Spelling Inspection Engine** — Build a background analysis worker that debounces text input, sends chunks to a fast LLM or grammar heuristic endpoint, and returns correction metadata.

```
Implement a client-side hook that debounces text changes in the TipTap editor by 1.5 seconds. When triggered, send the plain text payload to a Next.js API route. The API route must call Anthropic Claude Haiku with a strict JSON-mode system prompt instructing it to identify spelling errors, grammatical mistakes, and clarity issues, returning an array of fixes with start/end character offsets, error type, and suggested replacement.
```

4. **Inline Suggestion Cards & UI Overlays** — Render interactive underline decorations and floating tooltip cards inside the rich text editor when hovering over identified grammar mistakes.

```
Build a custom TipTap extension or floating overlay system that renders red/blue wavy underlines beneath text spans corresponding to the correction offsets returned by the inspection API. When a user clicks an underline, display a floating popover card showing the explanation, original text, and a one-click 'Accept' button that swaps the text in the editor state.
```

5. **Generative AI Rewrites & Tone Adjustment Panel** — Add a sidebar panel for generative AI features, allowing users to rewrite selected paragraphs for tone (formal, confident, friendly) or generate text from prompts.

```
Create a collapsible sidebar panel in the web editor named 'AI Assistant'. When text is highlighted in the editor, present quick action buttons: 'Improve clarity', 'Make professional', 'Shorten', and 'Expand'. Hook these buttons up to a streaming API route utilizing Anthropic Claude Sonnet that streams generated paragraph rewrites into a preview box with a 'Replace Selection' button.
```

### Cost vs paying

**Starting costs (one-time):**

- AI coding tool subscription: $20
- Total: ~$20 one-time

**Ongoing costs (monthly):**

- Anthropic API usage for text checks & rewrites: ~$5-15/mo
- Total: ~$5-15/mo

- Paying for the SaaS instead: $12/mo (Pro)
- Build time: 45 hours
- AI tool credits: $20 (Cursor/Claude Pro)
- Break-even: Never (built for learning and personal web editing)

## Sources

- [Crunchbase News - Grammarly Raises $1B from General Catalyst](https://news.crunchbase.com/venture/grammarly-raises-1b-general-catalyst/)
- [Grammarly Tech Blog - How Grammarly Tackles Hidden Complexity in Front-End Applications](https://www.grammarly.com/blog/engineering/)
- [Educative.io - A Practical Guide to the Grammarly System Design Interview](https://www.educative.io/blog/grammarly-system-design)